The present invention pertains generally to micro stepping motors, and more particularly to micro stepping motors used as electro-mechanical transducers in electronic analog wristwatches.
For purposes of facilitating an understanding and appreciation of the present invention, a brief description will first be given of one embodiment of micro stepping motor according to the invention disclosed and claimed in one of the earlier related applications listed above. One such embodiment of micro stepping motor used for driving the watch hands of an analog electronic wristwatch is shown in FIG. 1 and comprises a magnetic core 1 having wound thereon a coil 2, a stator 3 having opposite ends connected to the respective ends of the magnetic core 1 to define therewith a closed magnetic flux path, and a rotor 4 having a set of magnetic poles rotatably disposed within a hole in the stator 3. In this embodiment, the stator 3 has a constricted region of reduced cross-sectional area defined by concavely shaped connecting portions 3a, 3b and the hole in the stator for housing the rotor is located in the constricted region. A pair of recesses 5a, 5b are provided in the stator at diametrically opposite sites around the circumference of the hole for determining the rotor rest or stationary positions.
In the micro stepping motor embodiment shown in FIG. 1, the stator 3 and the core 1 are made of the same material thereby necessitating that the core 1 be made sufficiently large in total cross-section so as to obtain the required magnetic flux capacity during motor operation to apply the driving torque to the rotor 4 and to quickly magnetically saturate the stator connecting portions 3a, 3b. The cross-sectional area of the magnetic core 1 is designated S.sub.1. Therefore when the core and stator are composed of the same material, the core must have a sufficiently large cross-sectional area to meet the magnetic flux requirements for proper motor operation.
During operation, pulses of alternate polarity are applied to the coil 2 and the resulting magnetic fluxes generated by the coil flow in alternate directions through the stator 3. As the magnetic flux alternately flows through the stator constricted region defined by the connecting portions 3a, 3b, the narrow connecting portions quickly magnetically saturate and when in the saturated state function as air gaps diverting flux (leakage flux) to the stator hole to rotationally drive the rotor 4.